Reformat files according to new black validation

Change-Id: Ie550b7a41d14260080c9602df91d646e486e6884
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>
diff --git a/installers/charm/keystone/src/charm.py b/installers/charm/keystone/src/charm.py
index 39a7a00..446d2e0 100755
--- a/installers/charm/keystone/src/charm.py
+++ b/installers/charm/keystone/src/charm.py
@@ -248,9 +248,9 @@
     ):
         credentials_files_builder = FilesV3Builder()
         fernet_files_builder = FilesV3Builder()
-        for (key_id, _) in enumerate(credential_keys):
+        for key_id, _ in enumerate(credential_keys):
             credentials_files_builder.add_file(str(key_id), str(key_id), secret=True)
-        for (key_id, _) in enumerate(fernet_keys):
+        for key_id, _ in enumerate(fernet_keys):
             fernet_files_builder.add_file(str(key_id), str(key_id), secret=True)
         return credentials_files_builder.build(), fernet_files_builder.build()